This invention relates to a multiplex digital communication system or network of the type wherein communication is carried out by a higher-order multiplex digital signal into which a plurality of lower-order multiplex digital signals are multiplexed.
Such a multiplex digital communication system comprises a higher-order multiplex digital signal transmitter for transmitting a higher-order multiplex digital transmission signal to a transmission medium as a higher-order multiplex digital communication signal. The transmitter is accompanied by a plurality of lower-order multiplex digital signal transmitters or multiplexers. Each multiplexer produces a lower-order multiplex digital transmission signal. The higher-order transmitter multiplexes such lower-order multiplex digital transmission signals into the higher-order multiplex digital transmission signal.
A higher-order multiplex digital signal receiver receives the communication signal from the transmission medium as a higher-order multiplex digital reception signal. The receiver demultiplexes the reception signal into a plurality of lower-order multiplex digital reception signals, which are reproductions of the respective lower-order multiplex digital transmission signals. The receiver is accompanied by a plurality of lower-order multiplex digital signal receivers or demultiplexers. The higher-order receiver distributes the lower-order reception signals to the respective demultiplexers.
In a conventional multiplex digital communication system of the type described, a plurality of system numbers are assigned to the respective multiplexers and to the respective demultiplexers. The lower-order transmission signals are multiplexed in the order of the system numbers of the multiplexers for the respective lower-order transmission signals. The lower-order reception signals appear in the order of the system numbers. In order to correctly distribute the lower-order reception signals to the respective demultiplexers, exact frame synchronism must be established between the higher-order transmission and reception signals. The higher-order transmitter and receiver must therefore be designed so as to establish the frame synchronism between higher-rate signals and must comprise high-speed elements.